WHITE PLAINS, NY — Regulators investigating a fatal New York commuter train derailment recommended Tuesday that the railroad install recorders on its vehicles and new speed-limit signs along its tracks.

In a letter to the railroad’s new president, Joseph Giulietti, the National Transportation Safety Board said the recommendations arose from the Dec. 1 derailment in the Bronx that killed four people.

The railroad said it was studying the recommendations and working with the NTSB “to address questions about implementation of the report’s recommendations.”

Metro-North currently posts speed-limit signs where speeds are temporarily restricted, but not in every case where a restriction is permanent, the letter said. The permanent restrictions are listed in timetables, the NTSB said.

It said that on the curve where the train derailed — where a permanent 30 mph limit was in effect — no sign was posted until after the accident. The NTSB has found that the train hit the curve at 82 mph.

Erecting signs at all such locations “will alert train operating crews that speed restrictions are forthcoming and will comply with industry best practices,” the letter said.

The NTSB acknowledged that posting such signs “may not have prevented” the derailment.

It also called for recorders facing both into and out of all locomotives and cars with engineer cabs. The recorders would capture audio and images that could be used to investigate accidents and to monitor safety compliance, it said.

The board said it has been recommending such recorders for years and noted that the Federal Railroad Administration has not ordered implementation.

Chairman Deborah A.P. Hersman said in a news release, “Understanding what is happening inside the cab just prior to a crash can provide crucial information about how to prevent future accidents.”

Sen. Charles Schumer, D-N.Y., said, “This report should spur the FRA to move quickly on making these cameras mandatory, and prompt Metro-North to install them ASAP — and not wait until the feds require them to do so.”

The NTSB’s investigation of the derailment is ongoing, and its final report isn’t expected for months. Representatives of the train’s engineer, William Rockefeller, have said he may have lost focus at the controls in a momentary daze before the crash.